1. Field of the Invention
The present invention relates to an optical information recording medium on which information can be recorded, reproduced, erased and rewritten at high density and high speed by an optical method which includes irradiation with a laser beam. The present invention also relates to a method for manufacturing such a medium.
2. Description of the Prior Art
A phase change optical information recording medium utilizes a recording layer that causes a reversible phase change between a crystalline phase and an amorphous phase for recording, erasing and rewriting information. When this recording layer is irradiated by a laser beam of high power and is cooled rapidly, the irradiated portion becomes amorphous. In addition, when an amorphous portion of the recording layer is irradiated by a laser beam of low power and is cooled slowly, the irradiated portion becomes crystalline. Therefore, a phase change optical information recording medium can freely change the recording layer between an amorphous phase and crystalline phase when the recording layer is irradiated by a laser beam having a power that is modulated between a high power level and low power level. This optical information recording medium utilizes the difference of a reflection factor between the amorphous phase and the crystalline phase for recording information.
In order to increase the quantity of information that can be stored on the optical information recording medium, there is a basic method in which the density of a recording surface on the optical information recording medium is enhanced by shortening the wavelength of the laser beam or by enlarging the numerical aperture NA of an objective lens for condensing the laser beam so as to reduce the diameter of the laser spot. Recently, a blue laser having a wavelength of approximately 400 nm is beginning to have practical use. It has been proposed to reduce the diameter of the laser spot and improve the density of the recording surface by using a blue laser for the optical system for recording and reproducing of an optical information recording medium and by setting the numerical aperture NA of the objective lens of the optical system to a large value (approximately 0.60–0.85, like a DVD-RAM, for example). As a result of increasing the recording density, i.e. reducing the recording area per bit, the surface roughness of the film has had a large influence on disk characteristics.
A multilayered film as shown in FIG. 1, for example, is a typical structure of the phase change optical information recording medium (note that although FIG. 1 shows an embodiment of the present invention, it is referred to here for understanding the prior art). Namely, the optical information recording medium includes a substrate 1 of a resin such as polycarbonate or polymethyl methacrylate (PMMA) or a glass, on which a multilayered metal film 5, an upper dielectric layer 6, an upper interface layer 7, a recording layer 8, a lower interface layer 9 and a lower dielectric layer 10 are formed sequentially by sputtering, vapor deposition or the like.
ZnS—SiO2 is typically used as the material of the upper dielectric layer 6 and the lower dielectric layer 10. These dielectric layers have the functions of adjusting a reflection factor, an absorption ratio or the like of the disk by an interference effect of light, and protecting against evaporation of the recording layer or thermal damage to the substrate.
The upper interface layer 7 and the lower interface layer 9 have the functions of improving an erasing characteristic by promoting crystallization of the recording layer 8 and improving the repeating durability by preventing mutual diffusion of atoms between the recording layer 8 and the upper dielectric layer 6 as well as between the recording layer 8 and the lower dielectric layer 10.
The multilayered metal film 5 is a material having a high thermal conductivity, which improves efficiency in using light by reflecting the laser beam and works as a heat diffusion layer for rapidly dissipating heat generated in the recording layer 8. The multilayered metal film 5 is a single metal material such as Al or Ag having a high thermal conductivity, or a material containing one or more elements among them and one or more additive elements for improving humidity resistance or adjusting thermal conductivity or adjusting an optical reflection factor, an optical absorption ratio or an optical transmittance. More specifically, an alloy material such as Al—Cr, Al—Ti, Ag—Pd, Ag—Pd—Cu or Ag—Pd—Ti is used. In this way, the multilayered metal film 5 is a material that has the largest crystallinity among the layers, and so the surface roughness of the film mentioned above depends on the surface roughness of the multilayered metal film 5.
If a material containing Ag as a main component is used for the multilayered metal film 5, it has the advantage of a better cooling ability due to its higher thermal conductivity than a material containing Al as a main component. However, ZnS—SiO2 is typically used for the upper dielectric layer 6 as described above. Therefore, there is the problem of corrosion that may arise due to a reaction between Ag and S when the material of the multilayered metal film 5 containing Ag as a main component contacts the ZnS—SiO2 of the upper dielectric layer 6. That is, the material containing Ag as a main component has the problem of low resistance to corrosion.
In order to solve these problems of corrosion resistance and surface roughness of the film, a new structure has been proposed in which a barrier layer is disposed between the upper dielectric layer 6 and the metal layer 2 (see Japanese unexamined patent publication No. 2003-338083, for example). In addition, another structure has been proposed in which a metal film of Ag (30 nm), a high thermal conductivity film of Be (5 nm) and a metal film of Al (30 nm) are deposited in this order as the multilayered metal film 5 (see Japanese unexamined patent publication No. 2002-237098, pp 5–7 and FIG. 3). Here, the high thermal conductivity film of Be and the metal film of Al are inserted as the barrier layer for anti-corrosion, between the metal film of Ag and the upper dielectric layer of ZnS—SiO2. Therefore, the metal film of Ag cannot contact the ZnS—SiO2 layer, and so corrosion can be prevented. In addition, the metal material having a smaller crystal grain size than the material containing Al as a main component is formed before forming the metal film of Al in this structure, so the crystal grain size of Al can become smaller.
As a method for further reducing the crystal grain size of Al, a second element may be added. The added second element prevents the crystal grains of Al from growing, so they are smaller. However, if the second element is added to the conventional metal film of Al, the thermal conductivity is lowered, although the surface roughness is usually improved. Thus, the improvement in surface roughness is not compatible with the high cooling ability.